In a radio communication system, a radio transmitting apparatus for transmitting radio signals uses a power amplifier so as to amplify transmission signals. In a region where the amplitude of the input signal is small, the power amplifier has a linear characteristic such that the amplitude of the input signal and the amplitude of the output signal have a substantially linear relationship. On the other hand, in a region where the amplitude of the input signal is large, the power amplifier has a non-linear characteristic such that the amplitude of the input signal and the amplitude of the output signal have a non-linear relationship. In many cases, in order to use the power amplifier effectively, the power amplifier is operated not only in the linear region, but also in the non-linear region.
However, non-linear distortion of a transmission signal due to the non-linear amplification characteristic of the power amplifier may cause a reduction in the radio communication quality, such as power leakage from the desired frequency band to the adjacent frequency bands and the like. As one method for linearizing the non-linear amplification characteristic of the power amplifier, the radio transmitting apparatus performs distortion compensation using a pre-distortion scheme. For performing distortion compensation using the pre-distortion scheme, the radio transmitting apparatus is provided with a pre-distorter that serves as a linearizer.
The pre-distorter applies non-linear distortion having a characteristic opposite to the non-linear characteristic of the power amplifier to a transmission signal before input to the power amplifier. Then, the transmission signal to which the non-linear distortion having the opposite characteristic is applied passes through the power amplifier. Thus, it is possible to prevent the amplified transmission signal from being non-linearly distorted with respect to the transmission signal before input to the pre-distorter. For example, the pre-distorter applies to the transmission signal to be input to the power amplifier a compensation coefficient corresponding to the amplitude of the transmission signal (for example, multiplies the transmission signal by the compensation coefficient) such that the amplitude of the transmission signal becomes non-linear. The compensation coefficient corresponding to the amplitude of the transmission signal may be stored in a lookup table, for example.
However, it is not easy to accurately estimate the characteristic opposite to the non-linear characteristic of the power amplifier in advance. Therefore, in the distortion compensation system using the pre-distortion scheme, the compensation coefficient used by the pre-distorter is adaptively updated while operating the power amplifier. For example, the radio transmitting apparatus feeds back the output signal of the power amplifier. Then, an appropriate compensation coefficient is estimated on the basis of a feedback signal and a transmission signal before input to the power amplifier, and is set in the pre-distorter.
It is to be noted that there has been proposed a transmitter that includes an intermediate frequency amplifier upstream of a distortion compensation circuit. On the basis of a detected intermediate frequency signal before input to the distortion compensation circuit, the input signals to the distortion compensation circuit are controlled to be maintained at a constant level. There has also been proposed a radio communication apparatus for a radio communication system that includes an automatic gain controller upstream of a non-linear phase distortion compensator. The gain of a high-frequency signal to be input to the non-linear phase distortion compensator is controlled in accordance with an external signal. There has also been proposed an amplifying apparatus in which a variable attenuator is provided upstream of a pre-distortion circuit, and in which a power detector for detecting power leaked to the adjacent channels is provided downstream of an amplifier. This amplifying apparatus controls the attenuation amount of the variable attenuator on the basis of the leaked power.
Further, there has been proposed a distortion compensation apparatus in which a variable attenuator is provided between a multiplier for multiplying a baseband signal by a distortion compensation coefficient and a power amplifier. This distortion compensation apparatus controls the gain of the variable attenuator in accordance with the temperature and the frequency of an analog circuit. There has also been proposed a distortion compensation apparatus in which an attenuation circuit is provided between a distortion compensator and an amplifier circuit, and in which a detection circuit for detecting an output power of the amplifier circuit is provided downstream of the amplifier. This distortion compensation apparatus adjusts the attenuation amount of the attenuation circuit in accordance with the output power from the amplifier circuit. There has also been proposed a transmission amplifier that includes a power adjustment unit upstream of a non-linear distortion compensation unit. This transmission amplifier controls the gain of a symbol to be input to the non-linear distortion compensation unit on the basis of the average power of the symbol before input to the non-linear distortion compensation unit. There has also been proposed a radio transmitting apparatus that includes a gain adjustment unit in a distortion compensation unit. The gain adjustment unit corrects a distortion compensation coefficient in accordance with a power value of a feedback signal from a power amplifier.
These proposed techniques are disclosed, for example, in the following references: Japanese Laid-open Patent Publication No. 5-268117; Japanese Laid-open Patent Publication No. 2002-232328; Japanese Laid-open Patent Publication No. 2005-12419; International Publication Pamphlet No. WO2009/016686; Japanese Laid-open Patent Publication No. 2010-103834; Japanese Laid-open Patent Publication No. 2010-154017; and Japanese Laid-open Patent Publication No. 2010-278505.
The non-linear amplification characteristic of the power amplifier is not always constant, but varies with changes in temperature and the like, depending on the driving conditions of the power amplifier. Accordingly, when the average power of inputs to the power amplifier changes, the amplification characteristic of the power amplifier changes slightly. That is, the amplitude of an output signal after amplification corresponding to the amplitude of an input signal is dependent on the average power of input signals around that input signal. Such variation in the amplification characteristic of the power amplifier affects the training of the pre-distorter as described below.
It is assumed that, during the training, the compensation coefficient of the pre-distorter is repeatedly updated on the basis of feedback signals from the power amplifier. In this case, the updated compensation coefficient is calculated so as to match the amplification characteristic of the power amplifier before the update. However, when the compensation coefficient is updated, even when the average power of inputs to the pre-distorter remains the same, the average power of the outputs from the pre-distorter is changed from the average power before the update, so that the average power of the inputs to the power amplifier changes. Thus, the driving conditions of the power amplifier change, so that the amplification characteristic changes slightly. In the case where the training continues, the compensation coefficient is further updated so as to match the changed amplification characteristic of the power amplifier. Thus, the convergence of the compensation coefficient is delayed.